The present invention relates to electric meters containing electronic registers and, more particularly, to apparatus for providing battery backup to an internal clock in an electronic register effective for maintaining at least a clock time during a power outage.
Conventional electric meters employ an aluminum disk driven as a rotor of a small induction motor by an electric field at a speed proportional to the electric power being consumed by a load. Geared dials, or cyclometer discs, integrate the disk motion to indicate the total energy consumed, conventionally measured in kilowatt hours (one kilowatt hour equals one thousand watts of power consumption for one hour).
It is well accepted that the cost of electricity entails two components (1) the out-of-pocket cost (fuel, labor, etc.) for generating the electricity, and (2) the capital cost for providing the generating equipment. The out-of-pocket cost is recovered by a charge per unit of electricity consumed. The capital cost is related to the maximum load which the utility must supply. That is, synchronized peak loads from a substantial part of a utility's users can produce an instantaneous maximum load that requires a total generating capacity, representing a capital expenditure, far exceeding an average load. In order to provide a financial incentive for energy consumers to shift their power consumption from times when peak loads are known to occur to times of lower consumption, many rate-setting bodies permit utilities to charge higher rates at such peak times.
Thus, in addition to the above measurement of consumption, some electric meters contain means for separating the consumption into those parts occurring during predetermined peak and off-peak hours and for recording maximum demand during a predetermined period of time, in order to adjust billing according to such parameters. One type of electric meter is disclosed in U.S. Pat. No. 3,586,974, wherein a mechanical demand register records the power usage during a predetermined period of time and stores the value for later reading. The predetermined period of time may be, for example, the time between meter readings, or a period of time corresponding to the billing period of the utility providing the power. A clockwork mechanism restarts the mechanical demand register at the ends of regular demand intervals of, for example, a fraction of an hour, so that, at the end of the predetermined period, the stored value represents the highest value of power usage occurring during any one of the regular demand intervals in the predetermined period.
Greater flexibility is obtained using electronic acquisition, integration and processing of power usage. An electronic processor such as, for example, a microprocessor, may be employed to manage the acquisition, storage, processing and display of the usage and demand data. U.S. Pat. Nos. 4,179,654; 4,197,582; 4,229,795; 4,283,772; 4,301,508; 4,361,872 and 4,368,519, among others, illustrate the flexibility that electronic processing brings to power and energy usage measurement. Each of these electronic measurement devices includes means for producing an electronic signal having a characteristic such as, for example, a frequency or a pulse repetition rate, which is related to the rate of power usage. The electronic processor is substituted for at least part of the mechanical register of the prior art to keep track of the power usage during defined periods of time.
Various aspects of an electronic register which may benefit from the techniques of the present invention are disclosed in U.S. patent applications Ser. Nos. 599,684; 599,685; 599,736; 599,744; 599,683; 599,735; 599,743 and 599,742 all filed on Apr. 12, 1984; and Ser. Nos. 550,407 and 550,142 both filed on Nov. 10, 1983, the disclosures of which are herein incorporated by reference.
Electric meters which segregate usage and demand data according to time of use typically employ one mechanical or electronic register which accumulates the total energy usage, in addition to one or more additional registers which are active only between specific hours of the day or days of the week. More sophisticated time of use meters keep track of the day of the week, the season of the year and holidays. Such data is programmable for an extended period into the future such as, for example, 20 years. One such system, disclosed in U.S. Pat. No. 4,050,020, employs first and second clockwork registers. One of the registers is continuously active to accumulate total energy usage, while the other is enabled only at preselected times by an electronic circuit containing a built-in, or real-time, clock.
If such a real-time clock should stop for even a few relatively short periods of time, the relationship between data segregation and time of day, week, season and/or holiday would be damaged in a serious manner. As long as an electric meter receives line power, a real-time clock can be run from the line power. Line power occasionally fails, generally for relatively short periods of from a few minutes to an hour or two. It is conventional to provide a backup battery for maintaining operation of the real-time clock during such power failures.
In the prior art, a real-time clock for an electronic register employs NMOS devices. Such devices, besides consuming substantial power, require a power source capable of supplying about 5 volts DC. Since 5 VDC exceeds the voltage output of a conventional single cell, a multi-cell battery is required. In addition, due to their high power requirements, NMOS circuits need large battery capacity.
An electric meter generally has a design lifetime of at least 20 years. A battery, in contrast, reaches the end of its useful life in from about 2 to about 10 years under normal service. Battery lifetimes can be significantly shortened under adverse conditions such as, for example, an extended power outage lasting many hours or days. Rechargeable batteries, which are recharged by a built-in charger when the line power is restored, partially solve the capacity problem, but the number of charge-discharge cycles and the total life of such rechargeable batteries remain limited to less than the life of the electric meter. A rechargeable battery system for a real-time clock is disclosed in U.S. Pat. No. 4,199,717. The finite lifetime of a rechargeable battery requires that means be provided for battery replacement. The referenced patent does not contain specific disclosure for battery replacement.
Electric meters generally are built with a base mountable to a socket or terminals to sense the line voltage and current therethrough, a frame supporting and aligning the mechanical, electromechanical and electronic components, and a cover. The cover, which is generally transparent glass or plastic, provides a weather-tight and tamper-resistant enclosure for the frame and the apparatus mounted thereon. Security devices are generally used between the base and the socket, and between the cover and the base. A periodic requirement for removing the security devices, and then removing the cover for replacement of a battery, represents a significant labor burden on a utility.
The prior art, as exemplified by U.S. Pat. Nos. 4,075,561 and 4,297,635, addresses the problem of battery replacement from outside the meter cover by providing an access hatch in the cover of an electric meter for installation and removal of a battery from a unitary battery clip. Besides retaining the battery, the battery clip also makes electrical connection thereto. The disclosed battery clips do not provide for continuing battery power to critical components in the electric meter while the battery is partially withdrawn from the electric meter, or for permitting the battery to remain electrically connected to an electronic register module when the electronic register module is separated from the remainder of the electric meter.
More recent solid-state devices offer both lower power consumption and lower voltage requirements. A CMOS real-time clock circuit, for example, is capable of operation with a power consumption of about 200 microwatts at a voltage of about 2.5 volts. Thus, a 3-volt battery of modest total capacity is capable of powering a real-time clock for a period of, for example, 40 days. In addition, battery technology now offers long-life, non-rechargeable batteries such as, for example, lithium batteries, which have an improved power density and a lifetime of up to 10 years. This combination of low-power electronic circuits and long-life batteries of small size offers the ability to change the manner in which batteries are installed and replaced in electronic registers.
A favored technique for field service on an electronic register includes substitution of a known-good electronic register module for one that is apparently in a failed condition. In order to avoid zeroing the real-time clock, its parameters, and other data when the register is removed from the meter, the battery installation preferably should permit the battery to remain connected to the electronic register module when the electronic register module is separated from the remainder of the electric meter.